Chapter 6. Aspect Oriented Programming with Spring

6.1. Introduction

Aspect-Oriented Programming
(AOP) complements Object-Oriented Programming (OOP)
by providing another way of thinking about program structure. In addition
to classes, AOP gives you aspects. Aspects
enable modularization of concerns such as transaction management that
cut across multiple types and objects. (Such concerns are often
termed crosscutting concerns.)

One of the key components of Spring is the AOP
framework. While the Spring IoC container does not depend on AOP,
meaning you don't need to use AOP if you don't want to, AOP complements
Spring IoC to provide a very capable middleware solution.

Spring 2.0 AOP

Spring 2.0 introduces a simpler and more powerful
way of writing custom aspects using either a schema-based approach
or the @AspectJ annotation style. Both of these styles offer fully
typed advice and use of the AspectJ pointcut language, while still using
Spring AOP for weaving.

The Spring 2.0 schema and @AspectJ based AOP support is discussed in
this chapter. Spring 2.0 AOP remains fully backwards compatible with
Spring 1.2 AOP, and the lower-level AOP support offered by the Spring 1.2 APIs
is discussed in the following chapter.

AOP is used in the Spring Framework:

To provide declarative enterprise services, especially as a
replacement for EJB declarative services. The most important such
service is
declarative transaction management,
which builds on the Spring Framework's transaction abstraction.

To allow users to implement custom aspects, complementing their
use of OOP with AOP.

If you are interested only in generic declarative services or
other pre-packaged declarative middleware services such as pooling, you
don't need to work directly with Spring AOP, and can skip most of this
chapter.

6.1.1. AOP concepts

Let us begin by defining some central AOP concepts. These terms
are not Spring-specific. Unfortunately, AOP terminology is not
particularly intuitive; however, it would be even more confusing if
Spring used its own terminology.

Aspect: A modularization of a concern
that cuts across multiple objects. Transaction management is a good example
of a crosscutting concern in J2EE applications. In Spring AOP, aspects are
implemented using regular classes (the schema-based approach) or regular
classes annotated with the @Aspect annotation
(@AspectJ style).

Join point: A point during the execution of
a program, such as the execution of a method or the handling of
an exception. In Spring AOP, a join point always represents
a method execution. Join point information is available in advice
bodies by declaring a parameter of type org.aspectj.lang.JoinPoint.

Advice: Action taken by an aspect
at a particular join point. Different types of advice include
"around," "before" and "after" advice. Advice types are discussed
below. Many AOP frameworks, including Spring, model an advice as an
interceptor, maintaining a chain of
interceptors "around" the join point.

Pointcut: A predicate that matches join points.
Advice is associated with a pointcut expression and runs at any join
point matched by the pointcut (for example, the execution of a method with
a certain name). The concept of join points as matched by pointcut
expressions is central to AOP: Spring uses the AspectJ pointcut language by default.

Introduction: (Also known as an
inter-type declaration). Declaring additional methods or fields on
behalf of a type. Spring AOP allows you to introduce new interfaces
(and a corresponding implementation) to
any proxied object. For example, you could use an introduction to
make a bean implement an IsModified
interface, to simplify caching.

Target object: Object being advised
by one or more aspects. Also referred to as the advised
object. Since Spring AOP is implemented using runtime proxies, this object
will always be a proxied object.

AOP proxy: An object created by the AOP
framework in order to implement the aspect contracts (advise method
executions and so on). In the Spring Framework, an AOP proxy will be a JDK
dynamic proxy or a CGLIB proxy. Proxy creation is
transparent to users of the schema-based and @AspectJ styles of
aspect declaration introduced in Spring 2.0.

Weaving: Linking aspects with other
application types or objects to create an advised object. This can be
done at compile time (using the AspectJ compiler, for example), load time,
or at runtime. Spring AOP, like other pure Java AOP frameworks, performs
weaving at runtime.

Types of advice:

Before advice: Advice that executes
before a join point, but which does not have the ability to prevent
execution flow proceeding to the join point (unless it throws an
exception).

After returning advice: Advice to be
executed after a join point completes normally: for example, if a
method returns without throwing an exception.

After throwing advice: Advice to be executed if a
method exits by throwing an exception.

After (finally) advice: Advice to be executed regardless
of the means by which a join point exits (normal or exceptional return).

Around advice: Advice that surrounds a
join point such as a method invocation. This is the most powerful
kind of advice. Around advice can perform custom behavior before
and after the method invocation. It is also responsible for choosing
whether to proceed to the join point or to shortcut the advised method
execution by returning its own return value or throwing an exception.

Around advice is the most general kind of advice. Since Spring AOP,
like AspectJ, provides a full range of advice types, we recommend that you
use the least powerful advice type that can implement the required behavior.
For example, if you need only to update a cache with the return value of a
method, you are better off implementing an after returning advice than an
around advice, although an around advice can accomplish the same thing. Using
the most specific advice type provides a simpler programming model with less
potential for errors. For example, you do not need to invoke the
proceed() method on the JoinPoint
used for around advice, and hence cannot fail to invoke it.

In Spring 2.0, all advice parameters are statically typed, so that you
work with advice parameters of the appropriate type (the type of the return
value from a method execution for example) rather than
Object arrays.

The concept of join points, matched by pointcuts, is the key to AOP
which distinguishes it from older technologies offering only interception.
Pointcuts enable advice to be targeted independently of the Object-Oriented hierarchy.
For example, an around advice providing declarative transaction management can
be applied to a set of methods spanning multiple objects (such as all business
operations in the service layer).

6.1.2. Spring AOP capabilities and goals

Spring AOP is implemented in pure Java. There is no need for a
special compilation process. Spring AOP does not need to control the
class loader hierarchy, and is thus suitable for use in a J2EE web
container or application server.

Spring AOP currently supports only method execution join points
(advising the execution of methods on Spring beans). Field interception
is not implemented, although support for field interception could be added
without breaking the core Spring AOP APIs. If you need to advise field
access and update join points, consider a language such as AspectJ.

Spring AOP's approach to AOP differs from that of most other AOP
frameworks. The aim is not to provide the most complete AOP
implementation (although Spring AOP is quite capable); it is rather to
provide a close integration between AOP implementation and Spring IoC to
help solve common problems in enterprise applications.

Thus, for example, the Spring Framework's AOP functionality is normally
used in conjunction with the Spring IoC container. Aspects are configured
using normal bean definition syntax (although this allows powerful
"autoproxying" capabilities): this is a crucial difference from other AOP
implementations. There are some things you cannot do easily or
efficiently with Spring AOP, such as advise very fine-grained objects:
AspectJ is the best choice in such cases. However, our experience is that
Spring AOP provides an excellent solution to most problems in J2EE applications
that are amenable to AOP.

Spring AOP will never strive to compete with AspectJ to provide
a comprehensive AOP solution. We believe that both proxy-based frameworks like
Spring AOP and full-blown frameworks such as AspectJ are valuable, and that they
are complementary, rather than in competition. Spring 2.0 seamlessly integrates
Spring AOP and IoC with AspectJ, to enable all uses of AOP to be catered for within
a consistent Spring-based application architecture. This integration does not
affect the Spring AOP API or the AOP Alliance API: Spring AOP remains
backward-compatible. See the following chapter for a
discussion of the Spring AOP APIs.

Note

One of the central tenets of the Spring Framework is that of
non-invasiveness; this is the idea that you should not be forced to
introduce framework-specific classes and interfaces into your business/domain
model. However, in some places the Spring Framework does give you the option
to introduce Spring Framework-specific dependencies into your codebase: the
rationale in giving you such options is because in certain scenarios it
might be just plain easier to read or code some specific piece of functionality
in such a way. The Spring Framework (almost) always offers you the choice
though: you have the freedom to make an informed decision as to which option
best suits your particular use case or scenario.

One such choice that is relevant to this chapter is that of which AOP
framework (and which AOP style) to choose. You have the choice of AspectJ and/or
Spring AOP, and you also have the choice of either the @AspectJ annotation-style
approach or the Spring XML configuration-style approach. The fact that this chapter
chooses to introduce the @AspectJ-style approach first should not be taken as
an indication that the Spring team favors the @AspectJ annotation-style approach
over the Spring XML configuration-style.

6.1.3. AOP Proxies

Spring AOP defaults to using standard J2SE dynamic proxies
for AOP proxies. This enables any interface (or set of interfaces) to be
proxied.

Spring AOP can also use CGLIB proxies. This is necessary to proxy
classes, rather than interfaces. CGLIB is used by default if a business
object does not implement an interface. As it is good practice to
program to interfaces rather than classes, business classes normally will
implement one or more business interfaces. It is possible to
force the use of CGLIB,
in those (hopefully rare) cases where you need to advise a method that
is not declared on an interface, or where you need to pass a proxied object
to a method as a concrete type.

It is important to grasp the fact that Spring AOP is
proxy-based. See the section entitled
Section 6.6.1, “Understanding AOP proxies” for a thorough examination
of exactly what this implementation detail actually means.

6.2. @AspectJ support

@AspectJ refers to a style of declaring aspects as regular Java classes annotated with
Java 5 annotations. The @AspectJ style was introduced by the
AspectJ project as part of
the AspectJ 5 release. Spring 2.0 interprets the same annotations as AspectJ 5,
using a library supplied by AspectJ for pointcut parsing and matching. The AOP runtime
is still pure Spring AOP though, and there is no dependency on the AspectJ compiler
or weaver.

6.2.1. Enabling @AspectJ Support

To use @AspectJ aspects in a Spring configuration you need to enable Spring
support for configuring Spring AOP based on @AspectJ aspects, and
autoproxying beans based on whether or not they are advised by
those aspects. By autoproxying we mean that if Spring determines that a bean is advised
by one or more aspects, it will automatically generate a proxy for that bean to intercept
method invocations and ensure that advice is executed as needed.

The @AspectJ support is enabled by including the following element inside
your spring configuration:

You will also need two AspectJ libraries on the classpath of your application:
aspectjweaver.jar and
aspectjrt.jar. These libraries
are available in the 'lib' directory of an AspectJ installation (version
1.5.1 or later required), or in the 'lib/aspectj' directory of the
Spring-with-dependencies distribution.

6.2.2. Declaring an aspect

With the @AspectJ support enabled, any bean defined in your application context
with a class that is an @AspectJ aspect (has the @Aspect
annotation) will be automatically detected by Spring and used to configure Spring AOP.
The following example shows the minimal definition required for a not-very-useful
aspect:

A regular bean definition in the application context, pointing to a bean class
that has the @Aspect annotation:

Aspects (classes annotated with @Aspect) may have
methods and fields just like any other class. They may also contain pointcut, advice,
and introduction (inter-type) declarations.

Advising aspects

In Spring AOP, it is not possible to
have aspects themselves be the target of advice from other aspects.
The @Aspect annotation on a class marks it as an
aspect, and hence excludes it from auto-proxying.

6.2.3. Declaring a pointcut

Recall that pointcuts determine join points of interest, and thus enable us
to control when advice executes. Spring AOP only supports method execution
join points for Spring beans, so you can think of a pointcut as matching
the execution of methods on Spring beans. A pointcut declaration has two parts: a
signature comprising a name and any parameters, and a pointcut expression
that determines exactly which method executions we are interested in.
In the @AspectJ annotation-style of AOP, a pointcut signature is provided by a regular method
definition, and the pointcut expression is indicated using the
@Pointcut annotation (the method serving as the pointcut
signature must have a void return
type).

An example will help make this distinction between a pointcut signature and a
pointcut expression clear. The following example defines a pointcut named
'anyOldTransfer' that will match the execution of any method named
'transfer':

The pointcut expression that forms the value of the @Pointcut
annotation is a regular AspectJ 5 pointcut expression. For a full discussion of AspectJ's
pointcut language, see the
AspectJ Programming Guide
(and for Java 5 based extensions, the
AspectJ 5 Developers Notebook)
or one of the books on AspectJ such as “Eclipse AspectJ” by Colyer et. al.
or “AspectJ in Action” by Ramnivas Laddad.

6.2.3.1. Supported Pointcut Designators

Spring AOP supports the following AspectJ pointcut designators for use in pointcut
expressions:

Other pointcut types

The full AspectJ pointcut language supports additional pointcut designators
that are not supported in Spring. These are: call, initialization,
preinitialization, staticinitialization, get, set, handler, adviceexecution,
withincode, cflow, cflowbelow, if, @this, and @withincode.
Use of these pointcut designators in pointcut expressions interpreted by Spring AOP
will result in an IllegalArgumentException being thrown.

The set of pointcut designators supported by Spring AOP may be extended in
future releases both to support more of the AspectJ pointcut designators (e.g. "if"),
and potentially to support Spring specific designators such as "bean" (matching on
bean name).

execution - for matching method execution join points,
this is the primary pointcut designator you will use when working with Spring AOP

within - limits matching to join points within certain
types (simply the execution of a method declared within a matching type when using
Spring AOP)

this - limits matching to
join points (the execution of methods when using Spring AOP) where the bean
reference (Spring AOP proxy) is an instance of the given type

target - limits matching to join points (the execution
of methods when using Spring AOP) where the target object (application object
being proxied) is an instance of the given type

args - limits matching to
join points (the execution of methods when using Spring AOP) where the
arguments are instances of the given types

@target - limits matching
to join points (the execution of methods when using Spring AOP) where the
class of the executing object has an annotation of the given type

@args - limits matching to
join points (the execution of methods when using Spring AOP) where the
runtime type of the actual arguments passed have annotations of the given type(s)

@within - limits matching
to join points within types that have the given annotation (the execution of methods
declared in types with the given annotation when using Spring AOP)

@annotation - limits matching to join points where the
subject of the join point (method being executed in Spring AOP) has the given
annotation

Because Spring AOP limits matching to only method execution join points, the
discussion of the pointcut designators above gives a narrower definition than you will
find in the AspectJ programming guide. In addition, AspectJ itself has type-based
semantics and at an execution join point both 'this' and 'target' refer to the same
object - the object executing the method. Spring AOP is a proxy based system and
differentiates between the proxy object itself (bound to 'this') and the target
object behind the proxy (bound to 'target').

6.2.3.2. Combining pointcut expressions

Pointcut expressions can be combined using '&&', '||' and '!'. It is also possible
to refer to pointcut expressions by name. The following example shows three pointcut
expressions: anyPublicOperation (which matches if a method execution
join point represents the execution of any public method); inTrading (which matches
if a method execution is in the trading module), and tradingOperation
(which matches if a method execution represents any public method in the trading module).

It is a best practice to build more complex pointcut expressions out of smaller
named components as shown above. When referring to pointcuts by name, normal Java
visibility rules apply (you can see private pointcuts in the same type, protected
pointcuts in the hierarchy, public pointcuts anywhere and so on). Visibility does not
affect pointcut matching.

6.2.3.3. Sharing common pointcut definitions

When working with enterprise applications, you often want to refer to modules of the
application and particular sets of operations from within several aspects. We recommend
defining a "SystemArchitecture" aspect that captures common pointcut expressions
for this purpose. A typical such aspect would look as follows:

package com.xyz.someapp;
import org.aspectj.lang.annotation.Aspect;
import org.aspectj.lang.annotation.Pointcut;
@Aspect
public class SystemArchitecture {
/**
* A join point is in the web layer if the method is defined
* in a type in the com.xyz.someapp.web package or any sub-package
* under that.
*/
@Pointcut("within(com.xyz.someapp.web..*)")
public void inWebLayer() {}
/**
* A join point is in the service layer if the method is defined
* in a type in the com.xyz.someapp.service package or any sub-package
* under that.
*/
@Pointcut("within(com.xyz.someapp.service..*)")
public void inServiceLayer() {}
/**
* A join point is in the data access layer if the method is defined
* in a type in the com.xyz.someapp.dao package or any sub-package
* under that.
*/
@Pointcut("within(com.xyz.someapp.dao..*)")
public void inDataAccessLayer() {}
/**
* A business service is the execution of any method defined on a service
* interface. This definition assumes that interfaces are placed in the
* "service" package, and that implementation types are in sub-packages.
*
* If you group service interfaces by functional area (for example,
* in packages com.xyz.someapp.abc.service and com.xyz.def.service) then
* the pointcut expression "execution(* com.xyz.someapp..service.*.*(..))"
* could be used instead.
*/
@Pointcut("execution(* com.xyz.someapp.service.*.*(..))")
public void businessService() {}
/**
* A data access operation is the execution of any method defined on a
* dao interface. This definition assumes that interfaces are placed in the
* "dao" package, and that implementation types are in sub-packages.
*/
@Pointcut("execution(* com.xyz.someapp.dao.*.*(..))")
public void dataAccessOperation() {}
}

The pointcuts defined in such an aspect can be referred to anywhere that you
need a pointcut expression. For example, to make the service layer transactional, you
could write:

All parts except the returning type pattern (ret-type-pattern in the snippet above),
name pattern, and parameters pattern are optional.
The returning type pattern determines what the return type of the method must be in order
for a join point to be matched. Most frequently you will use * as the
returning type pattern, which matches any return type. A fully-qualified type name will
match only when the method returns the given type. The name pattern matches the method name.
You can use the * wildcard as all or part of a name pattern. The
parameters pattern is slightly more complex: () matches a method that takes
no parameters, whereas (..) matches any number of parameters (zero or more).
The pattern (*) matches a method taking one parameter of any type,
(*,String) matches a method taking two parameters, the first can be of
any type, the second must be a String. Consult the
Language Semantics section of the AspectJ Programming Guide for more
information.

Some examples of common pointcut expressions are given below.

the execution of any public method:

execution(public * *(..))

the execution of any method with a name beginning with "set":

execution(* set*(..))

the execution of any method defined by the AccountService interface:

execution(* com.xyz.service.AccountService.*(..))

the execution of any method defined in the service package:

execution(* com.xyz.service.*.*(..))

the execution of any method defined in the service package or a sub-package:

execution(* com.xyz.service..*.*(..))

any join point (method execution only in Spring AOP) within the service package:

within(com.xyz.service.*)

any join point (method execution only in Spring AOP) within the service package or a sub-package:

within(com.xyz.service..*)

any join point (method execution only in Spring AOP) where the proxy implements the AccountService interface:

this(com.xyz.service.AccountService)

'this' is more commonly used in a binding form :- see the following section
on advice for how to make the proxy object available in the advice body.

any join point (method execution only in Spring AOP) where the target object implements the AccountService interface:

target(com.xyz.service.AccountService)

'target' is more commonly used in a binding form :- see the following section
on advice for how to make the target object available in the advice body.

any join point (method execution only in Spring AOP) which takes a single parameter, and where the argument passed at runtime is Serializable:

args(java.io.Serializable)

'args' is more commonly used in a binding form :- see the following section
on advice for how to make the method arguments available in the advice body.

Note that the pointcut given in this example is different to
execution(* *(java.io.Serializable)): the args version matches if
the argument passed at runtime is Serializable, the execution version matches if the
method signature declares a single parameter of type Serializable.

any join point (method execution only in Spring AOP) where the target object has an @Transactional annotation:

@target(org.springframework.transaction.annotation.Transactional)

'@target' can also be used in a binding form :- see the following section
on advice for how to make the annotation object available in the advice body.

any join point (method execution only in Spring AOP) where the declared type of the target object has an @Transactional annotation:

@within(org.springframework.transaction.annotation.Transactional)

'@within' can also be used in a binding form :- see the following section
on advice for how to make the annotation object available in the advice body.

any join point (method execution only in Spring AOP) where the executing method has an @Transactional annotation:

@annotation(org.springframework.transaction.annotation.Transactional)

'@annotation' can also be used in a binding form :- see the following section
on advice for how to make the annotation object available in the advice body.

any join point (method execution only in Spring AOP) which takes a single parameter, and where the
runtime type of the argument passed has the @Classified annotation:

@args(com.xyz.security.Classified)

'@args' can also be used in a binding form :- see the following section
on advice for how to make the annotation object(s) available in the advice body.

6.2.4. Declaring advice

Advice is associated with a pointcut expression, and runs before, after, or
around method executions matched by the pointcut. The pointcut expression may be
either a simple reference to a named pointcut, or a pointcut expression declared
in place.

Note: it is of course possible to have multiple advice declarations, and other
members as well, all inside the same aspect. We're just showing a single advice declaration
in these examples to focus on the issue under discussion at the time.

Sometimes you need access in the advice body to the actual value that was returned. You
can use the form of @AfterReturning that binds the return
value for this:

The name used in the returning attribute must correspond to
the name of a parameter in the advice method. When a method execution returns, the
return value will be passed to the advice method as the corresponding argument value.
A returning clause also restricts matching to only those method
executions that return a value of the specified type (Object
in this case, which will match any return value).

Please note that it is not possible to
return a totally different reference when using after-returning advice.

6.2.4.3. After throwing advice

After throwing advice runs when a matched method execution exits by throwing
an exception. It is declared using the @AfterThrowing annotation:

Often you want the advice to run only when exceptions of a given type are
thrown, and you also often need access to the thrown exception in the advice
body. Use the throwing attribute to both restrict matching
(if desired, use Throwable as the exception type
otherwise) and bind the thrown exception to an advice parameter.

The name used in the throwing attribute must correspond to
the name of a parameter in the advice method. When a method execution exits by
throwing an exception, the
exception will be passed to the advice method as the corresponding argument value.
A throwing clause also restricts matching to only those method
executions that throw an exception of the specified type
(DataAccessException in this case).

6.2.4.4. After (finally) advice

After (finally) advice runs however a matched method execution exits.
It is declared using the @After annotation. After
advice must be prepared to handle both normal and exception return conditions.
It is typically used for releasing resources, etc.

6.2.4.5. Around advice

The final kind of advice is around advice. Around advice runs "around" a
matched method execution. It has the opportunity to do work both before and
after the method executes, and to determine when, how, and even if, the method
actually gets to execute at all. Around advice is often used if you need to
share state before and after a method execution in a thread-safe manner (starting
and stopping a timer for example). Always use the least powerful form of
advice that meets your requirements (i.e. don't use around advice if simple
before advice would do).

Around advice is declared using the @Around annotation. The first parameter
of the advice method must be of type ProceedingJoinPoint. Within the body of the
advice, calling proceed() on the ProceedingJoinPoint causes the
underlying method to execute. The proceed method may also be
called passing in an Object[] - the values in the array will be used as the arguments
to the method execution when it proceeds.

The behavior of proceed when called with an Object[] is a little different
than the behavior of proceed for around advice compiled by the AspectJ compiler. For
around advice written using the traditional AspectJ language, the number of arguments
passed to proceed must match the number of arguments passed to the around advice (not the
number of arguments taken by the underlying join point), and the value passed to proceed
in a given argument position supplants the original value at the join point for the entity
the value was bound to. (Don't worry if this doesn't make sense right now!) The
approach taken by Spring is simpler and a better match to its proxy-based, execution only
semantics. You only need to be aware of this difference if you compiling @AspectJ aspects
written for Spring and using proceed with arguments with the AspectJ compiler and weaver.
There is a way to write such aspects that is 100% compatible across both Spring AOP and
AspectJ, and this is discussed in the following section on advice parameters.

The value returned by the around advice will be the return value seen by the caller
of the method. A simple caching aspect for example could return a value from a cache
if it has one, and invoke proceed() if it does not. Note that proceed may be invoked
once, many times, or not at all within the body of the around advice, all of these
are quite legal.

6.2.4.6. Advice parameters

Spring 2.0 offers fully typed advice - meaning that you declare the parameters
you need in the advice signature (as we saw for the returning and throwing examples
above) rather than work with Object[] arrays all the time.
We'll see how to make argument and other contextual values available to the advice
body in a moment. First let's take a look at how to write generic advice that can find
out about the method the advice is currently advising.

6.2.4.6.1. Access to the current JoinPoint

Any advice method may declare as its first parameter, a parameter of
type org.aspectj.lang.JoinPoint (please note that
around advice is required to declare a first parameter of type
ProceedingJoinPoint, which is a subclass of
JoinPoint. The JoinPoint
interface provides a number of useful methods such as getArgs()
(returns the method arguments), getThis() (returns the proxy
object), getTarget() (returns the target object),
getSignature() (returns a description of the method that is
being advised) and toString() (prints a useful description of
the method being advised). Please do consult the Javadocs for full details.

6.2.4.6.2. Passing parameters to advice

We've already seen how to bind the returned value or exception value (using
after returning and after throwing advice). To make argument values available to
the advice body, you can use the binding form of args. If
a parameter name is used in place of a type name in an args expression, then the
value of the corresponding argument will be passed as the parameter value when the
advice is invoked. An example should make this clearer. Suppose you want to advise
the execution of dao operations that take an Account object as the first parameter,
and you need access to the account in the advice body. You could write the following:

The args(account,..) part of the pointcut expression serves
two purposes: firstly, it restricts matching to only those method executions where
the method takes at least one parameter, and the argument passed to that parameter
is an instance of Account; secondly, it makes the actual
Account object available to the advice via the
account parameter.

Another way of writing this is to declare a pointcut that "provides" the
Account object value when it matches a join point, and then
just refer to the named pointcut from the advice. This would look as follows:

The interested reader is once more referred to the AspectJ programming guide for
more details.

The proxy object (this), target object (target),
and annotations (@within, @target, @annotation, @args) can all
be bound in a similar fashion. The following example shows how you could match
the execution of methods annotated with an @Auditable
annotation, and extract the audit code.

6.2.4.6.3. Determining argument names

The parameter binding in advice invocations relies on matching names used
in pointcut expressions to declared parameter names in (advice and pointcut) method
signatures. Parameter names are not available through Java
reflection, so Spring AOP uses the following strategies to determine parameter
names:

If the parameter names have been specified by the user explicitly,
then the specified parameter names are used: both the advice and the pointcut
annotations have an optional "argNames" attribute which can be used to specify
the argument names of the annotated method - these argument names are
available at runtime. For example:

If an @AspectJ aspect has been compiled by the AspectJ compiler (ajc) then
there is no need to add the argNames attribute as the compiler will do this
automatically.

Using the 'argNames' attribute is a little clumsy, so if the
'argNames' attribute has
not been specified, then Spring AOP will look at the debug information for the
class and try to determine the parameter names from the local variable table. This
information will be present as long as the classes have been compiled with debug
information ('-g:vars' at a minimum). The consequences of
compiling with this flag on are: (1) your code will be slightly easier to understand
(reverse engineer), (2) the class file sizes will be very slightly bigger (typically
inconsequential), (3) the optimization to remove unused local variables will not be
applied by your compiler. In other words, you should encounter no difficulties building
with this flag on.

If the code has been compiled without the necessary debug information, then Spring AOP
will attempt to deduce the pairing of binding variables to parameters (for example,
if only one variable is bound in the pointcut expression, and the advice method only
takes one parameter, the pairing is obvious!). If the binding of variables is ambiguous
given the available information, then an AmbiguousBindingException
will be thrown.

If all of the above strategies fail then an
IllegalArgumentException will be thrown.

6.2.4.6.4. Proceeding with arguments

We remarked earlier that we would describe how to write a proceed call
with arguments that works consistently across Spring AOP
and AspectJ. The solution is simply to ensure that the advice signature binds
each of the method parameters in order. For example:

In many cases you will be doing this binding anyway (as in the example above).

6.2.4.7. Advice ordering

What happens when multiple pieces of advice all want to run at the same
join point? Spring AOP follows the same precedence rules as AspectJ to determine
the order of advice execution. The highest precedence advice runs first "on the way in"
(so given two pieces of before advice, the one with highest precedence runs
first). "On the way out" from a join point, the highest precedence advice runs last
(so given two pieces of after advice, the one with the highest precedence will run
second).

When two pieces of advice defined in different aspects both need
to run at the same join point, unless you specify otherwise the order of execution is
undefined. You can control the order of execution by specifying precedence. This is done in
the normal Spring way by either implementing the org.springframework.core.Ordered
interface in the aspect class or annotating it with the Order annotation.
Given two aspects, the aspect returning the lower value from
Ordered.getValue() (or the annotation value) has the higher precedence.

When two pieces of advice defined in the same aspect both need
to run at the same join point, the ordering is undefined (since there is no way to retrieve
the declaration order via reflection for javac-compiled classes). Consider collapsing
such advice methods into one advice method per joinpoint in each aspect class, or refactor
the pieces of advice into separate aspect classes - which can be ordered at the aspect level.

6.2.5. Introductions

Introductions (known as inter-type declarations in AspectJ) enable an aspect
to declare that advised objects implement a given interface, and to provide an implementation
of that interface on behalf of those objects.

An introduction is made using the @DeclareParents annotation. This annotation is
used to declare that matching types have a new parent (hence the name). For example, given
an interface UsageTracked, and an implementation of that interface
DefaultUsageTracked, the following
aspect declares that all implementors of service interfaces also implement the
UsageTracked interface.
(In order to expose statistics via JMX for example.)

The interface to be implemented is determined by the type of the annotated field. The
value attribute of the @DeclareParents annotation
is an AspectJ type pattern :- any bean of a matching type will implement the UsageTracked
interface. Note that in the before advice of the above example, service
beans can be directly used as implementations of the UsageTracked interface. If accessing
a bean programmatically you would write the following:

6.2.6. Aspect instantiation models

(This is an advanced topic, so if you are just starting out with AOP you can safely skip it
until later.)

By default there will be a single instance of each aspect within the application
context. AspectJ calls this the singleton instantiation model. It is possible to define
aspects with alternate lifecycles :- Spring supports AspectJ's perthis
and pertarget instantiation models (percflow, percflowbelow,
and pertypewithin are not currently supported).

A "perthis" aspect is declared by specifying a perthis clause
in the @Aspect annotation. Let's look at an example, and then
we'll explain how it works.

The effect of the 'perthis' clause is that one aspect
instance will be created for each unique service object executing a business
service (each unique object bound to 'this' at join points matched by the pointcut expression).
The aspect instance is created
the first time that a method is invoked on the service object. The aspect goes out
of scope when the service object goes out of scope. Before the aspect instance is created,
none of the advice within it executes. As soon as the aspect instance has been created,
the advice declared within it will execute at matched join points, but only when the service
object is the one this aspect is associated with. See the AspectJ programming guide for
more information on per-clauses.

The 'pertarget' instantiation model works in exactly the same way as perthis, but
creates one aspect instance for each unique target object at matched join points.

6.2.7. Example

Now that you have seen how all the constituent parts work, let's put them together
to do something useful!

The execution of business services can sometimes fail
due to concurrency issues (for example, deadlock loser).
If the operation is retried, it is quite likely to succeed next time round. For
business services where it is appropriate to retry in such conditions (idempotent
operations that don't need to go back to the user for conflict resolution), we'd like
to transparently retry the operation to avoid the client seeing a
PessimisticLockingFailureException. This is a requirement that clearly cuts across
multiple services in the service layer, and hence is ideal for implementing via an aspect.

Because we want to retry the operation, we will need to use around advice so
that we can call proceed multiple times. Here's how the basic aspect implementation
looks:

Note that the aspect implements the Ordered interface so we can set the precedence
of the aspect higher than the transaction advice (we want a fresh transaction
each time we retry). The maxRetries and order
properties will both be configured by Spring. The main action happens in the
doConcurrentOperation around advice. Notice that for the moment
we're applying the retry logic to all businessService()s. We
try to proceed, and if we fail with an PessimisticLockingFailureException
we simply try again unless we have exhausted all of our retry attempts.

and use the annotation to annotate the implementation of service operations.
The change to the aspect to only retry idempotent operations simply involves
refining the pointcut expression so that only @Idempotent operations match:

6.3. Schema-based AOP support

If you are unable to use Java 5, or simply prefer an XML-based format, then
Spring 2.0 also offers support for defining aspects using the new "aop" namespace
tags. The exact same pointcut expressions and advice kinds are supported as when
using the @AspectJ style, hence in this section we will focus on the new
syntax and refer the reader to the discussion in the previous
section (Section 6.2, “@AspectJ support”) for an understanding of writing pointcut
expressions and the binding of advice parameters.

Within your Spring configurations, all aspect and advisor elements must be
placed within an <aop:config> element (you can have more
than one <aop:config> element in an application context
configuration). An <aop:config> element can contain
pointcut, advisor, and aspect elements (note these must be declared in that order).

Warning

The <aop:config> style of configuration makes
heavy use of Spring's auto-proxying
mechanism. This can cause issues (such as advice not being woven) if you are
already using explicit auto-proxying via the use of
BeanNameAutoProxyCreator or suchlike. The recommended
usage pattern is to use either just the <aop:config>
style, or just the AutoProxyCreator style.

6.3.1. Declaring an aspect

Using the schema support, an aspect is simply a regular Java object
defined as a bean in your Spring application context. The state and behavior is
captured in the fields and methods of the object, and the pointcut and advice
information is captured in the XML.

An aspect is declared using the <aop:aspect> element, and the backing bean
is referenced using the ref attribute:

Note that the pointcut expression itself is using the same AspectJ pointcut
expression language as described in Section 6.2, “@AspectJ support”. If you are
using the schema based declaration style with Java 5, you can refer to named
pointcuts defined in types (@Aspects) within the pointcut expression, but this
feature is not available on JDK 1.4 and below (it relies on the Java 5 specific
AspectJ reflection APIs). On JDK 1.5 therefore, another way of defining the above
pointcut would be:

When combining pointcut sub-expressions, '&&' is awkward within an XML
document, and so the keywords 'and', 'or' and 'not' can be used in place
of '&&', '||' and '!' respectively.

Note that pointcuts defined in this way are referred to by their XML id, and
cannot define pointcut parameters. The named pointcut support in the schema based
definition style is thus more limited than that offered by the @AspectJ style.

6.3.3. Declaring advice

The same five advice kinds are supported as for the @AspectJ style, and they
have exactly the same semantics.

6.3.3.1. Before advice

Before advice runs before a matched method execution.
It is declared inside an <aop:aspect> using
the <aop:before> element.

As we noted in the discussion of the @AspectJ style, using named pointcuts
can significantly improve the readability of your code.

The method attribute identifies a method (doAccessCheck) that
provides the body of the advice. This method must be defined for the bean referenced
by the aspect element containing the advice. Before a data access operation is
executed (a method execution join point matched by the pointcut expression), the
"doAccessCheck" method on the aspect bean will be invoked.

6.3.3.2. After returning advice

After returning advice runs when a matched method execution completes normally.
It is declared inside an <aop:aspect> in
the same way as before advice. For example:

Just as in the @AspectJ style, it is possible to get hold of the return value within
the advice body. Use the returning attribute to specify the name of the parameter to which
the return value should be passed:

The doAccessCheck method must declare a parameter named retVal. The
type of this parameter constrains matching in the same way as described for @AfterReturning.
For example, the method signature may be declared as:

public void doAccessCheck(Object retVal) {...

6.3.3.3. After throwing advice

After throwing advice executes when a matched method execution exits by throwing
an exception. It is declared inside an <aop:aspect> using
the after-throwing element:

Just as in the @AspectJ style, it is possible to get hold of the thrown exception within
the advice body. Use the throwing attribute to specify the name of the parameter to which
the exception should be passed:

The doRecoveryActions method must declare a parameter named dataAccessEx.
The type of this parameter constrains matching in the same way as described for
@AfterThrowing.
For example, the method signature may be declared as:

public void doRecoveryActions(DataAccessException dataAccessEx) {...

6.3.3.4. After (finally) advice

After (finally) advice runs however a matched method execution exits. It is declared
using the after element:

6.3.3.5. Around advice

The final kind of advice is around advice. Around advice runs "around" a
matched method execution. It has the opportunity to do work both before and
after the method executes, and to determine when, how, and even if, the method
actually gets to execute at all. Around advice is often used if you need to
share state before and after a method execution in a thread-safe manner (starting
and stopping a timer for example). Always use the least powerful form of
advice that meets your requirements; don't use around advice if simple
before advice would do.

Around advice is declared using the aop:around element.
The first parameter of the advice method must be of type
ProceedingJoinPoint. Within the body of the
advice, calling proceed() on the
ProceedingJoinPoint causes the
underlying method to execute. The proceed method may also be
calling passing in an Object[] - the values in the array will be used as the arguments
to the method execution when it proceeds. See Section 6.2.4.5, “Around advice”
for notes on calling proceed with an Object[].

6.3.3.6. Advice parameters

The schema based declaration style supports fully typed advice in the same way as
described for the @AspectJ support - by matching pointcut parameters by name
against advice method parameters. See Section 6.2.4.6, “Advice parameters”
for details.

If you wish to explicity specify argument names for the advice methods (not
relying on either of the detection strategies previously described) then this is
done using the arg-names attribute of the advice element. For
example:

Next up is the aspect. Notice the fact that the
profile(..) method accepts a number of strongly-typed parameters,
the first of which happens to be the join point used to proceed with the method call: the
presence of this parameter is an indication that the profile(..)
is to be used as around advice:

6.3.3.7. Advice ordering

When multiple advice needs to execute at the same join point (executing
method) the ordering rules are as described in Section 6.2.4.7, “Advice ordering”.
The precedence between aspects is determined by either adding the Order
annotation to the bean backing the aspect or by having the bean implement the
Ordered interface.

6.3.4. Introductions

Introductions (known as inter-type declarations in AspectJ) enable an aspect
to declare that advised objects implement a given interface, and to provide an implementation
of that interface on behalf of those objects.

An introduction is made using the aop:declare-parents element
inside an aop:aspect This element is
used to declare that matching types have a new parent (hence the name). For example, given
an interface UsageTracked, and an implementation of that
interface DefaultUsageTracked, the following
aspect declares that all implementors of service interfaces also implement the
UsageTracked interface. (In order to expose statistics
via JMX for example.)

The interface to be implemented is determined by implement-interface
attribute. The value of the
types-matching attribute is an AspectJ type pattern :-
any bean of a matching type will implement the UsageTracked
interface. Note that in the before advice of the above example, service
beans can be directly used as implementations of the UsageTracked interface. If accessing
a bean programmatically you would write the following:

6.3.5. Aspect instantiation models

The only supported instantiation model for schema-defined aspects is the
singleton model. Other instantiation models may be supported in future releases.

6.3.6. Advisors

The concept of "advisors" is brought forward from the AOP support defined
in Spring 1.2 and does not have a direct equivalent in AspectJ. An advisor is
like a small self-contained aspect that has a single piece of advice. The advice
itself is represented by a bean, and must implement one of the advice
interfaces described in Section 7.3.2, “Advice types in Spring”. Advisors can
take advantage of AspectJ pointcut expressions though.

Spring 2.0 supports the advisor concept with the <aop:advisor>
element. You will most commonly see it used in conjunction with transactional advice,
which also has its own namespace support in Spring 2.0. Here's how it looks:

As well as the pointcut-ref attribute used in the above
example, you can also use the pointcut attribute to define a
pointcut expression inline.

To define the precedence of an advisor so that the advice can participate
in ordering, use the order attribute to define the
Ordered value of the advisor.

6.3.7. Example

Let's see how the concurrent locking failure retry example from
Section 6.2.7, “Example” looks when rewritten using the schema support.

The execution of business services can sometimes fail
due to concurrency issues (for example, deadlock loser).
If the operation is retried, it is quite likely it will succeed next time round. For
business services where it is appropriate to retry in such conditions (idempotent
operations that don't need to go back to the user for conflict resolution), we'd like
to transparently retry the operation to avoid the client seeing a
PessimisticLockingFailureException. This is a requirement that clearly cuts across
multiple services in the service layer, and hence is ideal for implementing via an aspect.

Because we want to retry the operation, we'll need to use around advice so
that we can call proceed multiple times. Here's how the basic aspect implementation
looks (it's just a regular Java class using the schema support):

Note that the aspect implements the Ordered interface so we can set the precedence
of the aspect higher than the transaction advice (we want a fresh transaction
each time we retry). The maxRetries and order
properties will both be configured by Spring. The main action happens in the
doConcurrentOperation around advice method. We
try to proceed, and if we fail with a PessimisticLockingFailureException
we simply try again unless we have exhausted all of our retry attempts.

This class is identical to the one used in the @AspectJ example, but
with the annotations removed.

Notice that for the time being we assume that all business services
are idempotent. If this is not the case we can refine the aspect so that it only
retries genuinely idempotent operations, by introducing an
Idempotent annotation:

and using the annotation to annotate the implementation of service operations.
The change to the aspect to only retry idempotent operations simply involves
refining the pointcut expression so that only @Idempotent operations match:

6.4. Choosing which AOP declaration style to use

Once you have decided that an aspect is the best approach for implementing a
given requirement, how do you decide between using Spring AOP or AspectJ,
and between the Aspect language (code) style, @AspectJ annotation style, and
the XML style? These decisions are influenced by a number of factors including
application requirements, development tools, and team familiarity with AOP.

6.4.1. Spring AOP or full AspectJ?

Use the simplest thing that can work. Spring AOP is simpler than using
full AspectJ as there is no requirement to introduce the AspectJ
compiler / weaver into your development and build processes.
If you only need to advise the execution of operations on Spring beans,
then Spring AOP is the right choice. If you need to advise domain objects,
or any other object not managed by the Spring container, then you will
need to use AspectJ. You will also need to use AspectJ if you wish to
advise join points other than simple method executions (for example, call
join points, field get or set join points, and so on).

When using AspectJ, you have the choice of the AspectJ language syntax
(also known as the "code style") or the @AspectJ annotation style.
If aspects play a large role in your design, and you are able to use the
AspectJ Development Tools (AJDT)
in Eclipse, then the AspectJ language syntax is the preferred option: it is cleaner
and simpler because the language was purposefully designed for writing aspects.
If you are not using Eclipse, or have only a few aspects that do not play a
major role in your application, then you may want to consider using the @AspectJ
style and sticking with a regular Java compilation in your IDE, and adding an aspect
weaving (linking) phase to your build scripts.

6.4.2. @AspectJ or XML for Spring AOP?

The XML style will be most familiar to existing Spring users. It can be
used with any JDK level (referring to named pointcuts from within
pointcut expressions does still require Java 5 though) and is backed
by genuine POJOs. When using AOP as a tool to configure enterprise
services (a good test is whether you consider the pointcut expression
to be a part of your configuration you might want to change independently)
then XML can be a good choice. With the XML style it is arguably
clearer from your configuration what aspects are present in the system.

The XML style has two disadvantages. Firstly it does not fully encapsulate
the implementation of the requirement it addresses in a single place.
The DRY principle says that there should be a single, unambiguous, authoritative
representation of any piece of knowledge within a system. When using the
XML style, the knowledge of how a requirement is implemented is split across
the declaration of the backing bean class, and the XML in the configuration
file. When using the @AspectJ style there is a single module - the aspect - in
which this information is encapsulated. Secondly, the XML style is more
limited in what in can express than the @AspectJ style: only the "singleton"
aspect instantiation model is supported, and it is not possible to combine
named pointcuts declared in XML. For example, in the @AspectJ style we
can write something like:

The downside of the XML approach becomes evident in this case because I
cannot define the 'accountPropertyAccess' pointcut by
combining these definitions.

The @AspectJ style supports additional instantiation models, and richer
pointcut composition. It has the advantage of keeping the aspect as a
modular unit. It also has the advantage the @AspectJ aspects can be
understood both by Spring AOP and by AspectJ - so if you later decide
you need the capabilities of AspectJ to implement additional requirements
then it is very easy to migrate to an AspectJ based approach.

So much for the pros and cons of each style then: which is best? If you are not using Java 5
(or above) then clearly the XML-style is appropriate because it is the only option available to you.
If you are using Java 5+, then you really will have to come to your own decision as to which style suits
you best. In the experience of the Spring team, we advocate the use of the @AspectJ style whenever
there are aspects that do more than simple "configuration" of enterprise services. If you are writing,
have written, or have access to an aspect that is not part of the business contract of a particular
class (such as a tracing aspect), then the XML-style is better.

6.5. Mixing aspect types

It is perfectly possible to mix @AspectJ style aspects using the autoproxying
support, schema-defined <aop:aspect> aspects,
<aop:advisor> declared advisors and even proxies and
interceptors defined using the Spring 1.2 style in the same configuration. All of
these are implemented using the same underlying support mechanism and will co-exist
without any difficulty.

6.6. Proxying mechanisms

Spring AOP uses either JDK dynamic proxies or CGLIB to create the proxy for a given
target object. (JDK dynamic proxies are preferred whenever you have a choice).

If the target object to be proxied implements at least one interface then a JDK
dynamic proxy will be used. All of the interfaces implemented by the target type will be
proxied. If the target object does not implement any interfaces then a CGLIB proxy
will be created.

If you want to force the use of CGLIB proxying (for example, to proxy every
method defined for the target object, not just those implemented by its interfaces)
you can do so. However, there are some issues to consider:

final methods cannot be advised, as they cannot be overriden.

You will need the CGLIB 2 binaries on your classpath, whereas dynamic proxies are available with the JDK.
Spring will automatically warn you when it needs CGLIB and the CGLIB library classes are not found on the
classpath.

The constructor of your proxied object will be called twice. This is a natural consequence of the CGLIB
proxy model whereby a subclass is generated for each proxied object. For each proxied instance, two objects are
created: the actual proxied object and an instance of the subclass that implements the advice. This behavior
is not exhibited when using JDK proxies. Usually, calling the constructor of the proxied type twice, is not an
issue, as there are usually only assignments taking place and no real logic is implemented in the constructor.

To force the use of CGLIB proxies set the value of the proxy-target-class
attribute of the <aop:config> element to true:

To force CGLIB proxying when using the @AspectJ autoproxy support, set the
'proxy-target-class' attribute of the
<aop:aspectj-autoproxy> element to true:

<aop:aspectj-autoproxy proxy-target-class="true"/>

Note

Multiple <aop:config/> sections are collapsed into a single unified auto-proxy
creator at runtime, which applies the strongest proxy settings that any of the
<aop:config/> sections (typically from different XML bean definition files) specified.
This also applies to the <tx:annotation-driven/> and
<aop:aspectj-autoproxy/> elements.

To be clear: using 'proxy-target-class="true"' on
<tx:annotation-driven/>, <aop:aspectj-autoproxy/> or
<aop:config/> elements will force the use of CGLIB proxies
for all three of them.

6.6.1. Understanding AOP proxies

Spring AOP is proxy-based. It is vitally important that you grasp
the semantics of what that last statement actually means before you write your own aspects or
use any of the Spring AOP-based aspects supplied with the Spring Framework.

Consider first the scenario where you have a plain-vanilla, un-proxied,
nothing-special-about-it, straight object reference, as illustrated by the following code snippet.

The key thing to understand here is that the client code inside the
main(..) of the Main class
has a reference to the proxy. This means that method calls on that object
reference will be calls on the proxy, and as such the proxy will be able to delegate to
all of the interceptors (advice) that are relevant to that particular method call.
However, once the call has finally reached the target object, the SimplePojo
reference in this case, any method calls that it may make on itself, such as
this.bar() or this.foo(), are going to be
invoked against the this reference, and not
the proxy. This has important implications. It means that self-invocation is not
going to result in the advice associated with a method invocation getting a chance to execute.

Okay, so what is to be done about this? The best approach (the term best is used loosely here)
is to refactor your code such that the self-invocation does not happen. For sure, this does entail
some work on your part, but it is the best, least-invasive approach. The next approach is
absolutely horrendous, and I am almost reticent to point it out precisely because it is so
horrendous. You can (choke!) totally tie the logic within your class to Spring AOP by doing
this:

This totally couples your code to Spring AOP, and it makes the class
itself aware of the fact that it is being used in an AOP context, which flies in the face of AOP.
It also requires some additional configuration when the proxy is being created:

Finally, it must be noted that AspectJ does not have this self-invocation issue because it is
not a proxy-based AOP framework.

6.7. Programmatic creation of @AspectJ Proxies

In addition to declaring aspects in your configuration using either
<aop:config> or <aop:aspectj-autoproxy>,
it is also possible programmatically to create proxies that advise target objects. For the
full details of Spring's AOP API, see the next chapter. Here we want to focus on the ability
to automatically create proxies using @AspectJ aspects.

The class org.springframework.aop.aspectj.annotation.AspectJProxyFactory
can be used to create a proxy for a target object that is advised by one or more
@AspectJ aspects. Basic usage for this class is very simple, as illustrated below. See
the Javadocs for full information.

// create a factory that can generate a proxy for the given target object
AspectJProxyFactory factory = new AspectJProxyFactory(targetObject);
// add an aspect, the class must be an @AspectJ aspect
// you can call this as many times as you need with different aspects
factory.addAspect(SecurityManager.class);
// you can also add existing aspect instances, the type of the object supplied must be an @AspectJ aspect
factory.addAspect(usageTracker);
// now get the proxy object...
MyInterfaceType proxy = factory.getProxy();

6.8. Using AspectJ with Spring applications

Everything we've covered so far in this chapter is pure Spring AOP. In this section,
we're going to look at how you can use the AspectJ compiler/weaver instead of or in
addition to Spring AOP if your needs go beyond the facilities offered by Spring AOP
alone.

6.8.1. Using AspectJ to dependency inject domain objects with Spring

The Spring container instantiates and configures beans defined in your
application context. It is also possible to ask a bean factory to configure
a pre-existing object given the name of a bean definition
containing the configuration to be applied. The spring-aspects.jar
contains an annotation-driven aspect that exploits this capability to allow dependency-injection
of any object. The support is intended to be used for objects created
outside of the control of any container. Domain objects
often fall into this category: they may be created programmatically using the
new operator, or by an ORM tool as a result of a database query.

The @Configurable annotation marks a class as eligible for
Spring-driven configuration. In the simplest case it can be used just as a
marker annotation:

When used as a marker interface in this way, Spring will configure
new instances of the annotated type (Account in this case) using a
prototypical bean definition with the same name as the fully-qualified
type name (com.xyz.myapp.domain.Account). Since the
default name for a bean is the fully-qualified name of its type, a
convenient way to declare the prototype definition is simply to omit the
id attribute:

Spring will now look for a bean definition named "account" and
use that as a prototypical definition to configure new Account
instances.

You can also use autowiring to avoid having to specify a prototypical
bean definition at all. To have Spring apply autowiring use the
autowire property of the @Configurable annotation: specify either
@Configurable(autowire=Autowire.BY_TYPE) or
@Configurable(autowire=Autowire.BY_NAME for autowiring
by type or by name respectively.

Finally you can enable Spring dependency checking for the object references
in the newly created and configured object by using the dependencyCheck
attribute (for example:
@Configurable(autowire=Autowire.BY_NAME,dependencyCheck=true) ).
If this attribute is set to true, then Spring will validate after configuration that all properties
(that are not primitives or collections) have been set.

Using the annotation on its own does nothing of course. It's the
AnnotationBeanConfigurerAspect in spring-aspects.jar that
acts on the presence of the annotation. In essence the aspect says "after
returning from the initialization of a new object of a type with the
@Configurable annotation, configure the newly created object using Spring
in accordance with the properties of the annotation". For this to work the
annotated types must be woven with the AspectJ weaver - you can either
use a build-time ant or maven task to do this (see for example the
AspectJ
Development Environment Guide) or load-time weaving (see Section 6.8.4, “Using AspectJ Load-time weaving (LTW) with Spring applications”).

The AnnotationBeanConfigurerAspect itself needs
configuring by Spring (in order to obtain a reference to the bean factory that
is to be used to configure new objects). The Spring AOP namespace defines a
convenient tag for doing this. Simply include the following in your
application context configuration:

<aop:spring-configured/>

If you are using the DTD instead of schema, the equivalent definition is:

Instances of @Configurable objects created before
the aspect has been configured will result in a warning being issued to the
log and no configuration of the object taking place. An example might be
a bean in the Spring configuration that creates domain objects when it is
initialized by Spring. In this case you can use the "depends-on" bean
attribute to manually specify that the bean depends on the
configuration aspect.

6.8.1.1. Unit testing @Configurable objects

One of the goals of the @Configurable support is to enable independent
unit testing of domain objects without the difficulties associated with
hard-coded lookups. If @Configurable types have not been woven by AspectJ
then the annotation has no affect during unit testing, and you can simply
set mock or stub property references in the object under test and proceed
as normal. If @Configurable types have been woven by
AspectJ then you can still unit test outside of the container as normal,
but you will see a warning message each time that you construct an
@Configurable object indicating that it has not been configured by Spring.

6.8.1.2. Working with multiple application contexts

The AnnotationBeanConfigurerAspect used to implement
the @Configurable support is an AspectJ singleton aspect. The scope of a
singleton aspect is the same as the scope of static members, that is to say there is
one aspect instance per classloader that defines the type. This means that if
you define multiple application contexts within the same classloader hierarchy
you need to consider where to define the <aop:spring-configured/> bean and where to place
spring-aspects.jaron the classpath.

Consider a typical Spring web-app configuration with a shared parent application
context defining common business services and everything needed to support
them, and one child application context per servlet containing definitions
particular to that servlet. All of these contexts will co-exist within the
same classloader hierarchy, and so the AnnotationBeanConfigurerAspect
can only hold a reference to one of them. In this case we recommend
defining the <aop:spring-configured/> bean in the
shared (parent) application context: this defines the services that you are
likely to want to inject into domain objects. A consequence is that you cannot
configure domain objects with references to beans defined in the child
(servlet-specific) contexts using the @Configurable mechanism
(probably not something you want to do anyway!).

When deploying multiple web-apps within the same container, ensure that each
web-application loads the types in spring-aspects.jar using its own classloader
(for example, by placing spring-aspects.jar
in 'WEB-INF/lib'). If spring-aspects.jar
is only added to the container wide classpath (and hence loaded by the shared
parent classloader), all web applications will share the same aspect instance
which is probably not what you want.

6.8.2. Other Spring aspects for AspectJ

In addition to the @Configurable support,
spring-aspects.jar contains an
AspectJ aspect that can be used to drive Spring's transaction management for
types and methods annotated with the @Transactional annotation.
This is primarily intended for users who want to use Spring's transaction support outside of the Spring container.

The aspect that interprets @Transactional annotations is the
AnnotationTransactionAspect. When using this
aspect, you must annotate the implementation class
(and/or methods within that class), not the interface
(if any) that the class implements. AspectJ follows Java's rule that annotations on
interfaces are not inherited.

A @Transactional annotation on a class specifies the default transaction
semantics for the execution of any public operation in the class.

A @Transactional annotation on a method within the class overrides the
default transaction semantics given by the class annotation (if present).
Methods with public, protected, and default visibility may all be annotated.
Annotating protected and default visibility methods directly is the only way
to get transaction demarcation for the execution of such operations.

For AspectJ programmers that want to use the Spring configuration and
transaction management support but don't want to (or can't) use annotations,
spring-aspects.jar also contains abstract aspects you can extend to provide
your own pointcut definitions. See the Javadocs for
AbstractBeanConfigurerAspect and
AbstractTransactionAspect for more information. As an example,
the following excerpt shows how you could write an aspect to configure
all instances of objects defined in the domain model
using prototypical bean definitions that match the fully-qualified class
names:

6.8.3. Configuring AspectJ aspects using Spring IoC

When using AspectJ aspects with Spring applications, it's natural to want
to configure such aspects using Spring. The AspectJ runtime itself is responsible
for aspect creation, and the means of configuring the AspectJ created aspects via
Spring depends on the AspectJ instantiation model (per-clause) used by the aspect.

The majority of AspectJ aspects are
singleton aspects. Configuration of these aspects is very
easy, simply create a bean definition referencing the aspect type as normal, and
include the bean attribute 'factory-method="aspectOf"'. This
ensures that Spring obtains the aspect instance by asking AspectJ for it rather
than trying to create an instance itself. For example:

For non-singleton aspects, the easiest way to configure them is to create
prototypical bean definitions and annotate use the @Configurable support from
spring-aspects.jar to configure the aspect instances once they have bean created
by the AspectJ runtime.

If you have some @AspectJ aspects that you want to weave with AspectJ
(for example, using load-time weaving for domain model types) and other @AspectJ
aspects that you want to use with Spring AOP, and these aspects are all configured
using Spring, then you'll need to tell the Spring AOP @AspectJ autoproxying support
which subset of the @AspectJ aspects defined in the configuration should be used
for autoproxying. You can do this by using one or more <include/>
elements inside the <aop:aspectj-autoproxy/> declaration.
Each include element specifies a name pattern, and only beans with names matched
by at least one of the patterns will be used for Spring AOP autoproxy configuration:

6.8.4. Using AspectJ Load-time weaving (LTW) with Spring applications

Load-time weaving (or LTW) refers to the process of weaving AspectJ aspects
with an application's class files as they are loaded into the VM. For full details
on configuring load-time weaving with AspectJ, see the
LTW section of the AspectJ Development Environment Guide
. We will focus here on the essentials of configuring load-time weaving
for Spring applications running on Java 5.

Load-time weaving is controlled by defining a file 'aop.xml' in
the META-INF directory. AspectJ automatically looks for all 'META-INF/aop.xml' files
visible on the classpath and configures itself based on the aggregation of their
content.

The <include/> element tells AspectJ which set of types
should be included in the weaving process. Use the package prefix for your
application followed by "..*" (meaning '... and any type defined in a subpackage
of this') as a good default. Using the include element is important as otherwise
AspectJ will look at every type loaded in support of your application (including
all the Spring library classes and many more besides). Normally you don't want
to weave these types and don't want to pay the overhead of AspectJ attempting
to match against them.

To get informational messages in your log file regarding the activity of the
load-time weaver, add the following options to the weaver element:

Finally, to control exactly which aspects are used, you can use the
aspects element. By default all defined aspects are used for
weaving (spring-aspects.jar contains a META-INF/aop.xml file that defines the
configuration and transaction aspects). If you were using spring-aspects.jar, but
only want the configuration support and not the transaction support you could
specify this as follows:

6.9. Further Resources

The book Eclipse AspectJ by Adrian Colyer et. al. (Addison-Wesley,
2005) provides a comprehensive introduction and reference for the AspectJ language.

The excellent AspectJ in Action by Ramnivas Laddad (Manning, 2003)
comes highly recommended as an introduction to AOP; the focus of the book is on
AspectJ, but a lot of general AOP themes are explored in some depth.